Curable resin compositions constituted by epoxy resins and curing agents therefor are widely used as semiconductor sealing materials and insulating materials for printed wiring substrates since the cured products thereof have good heat resistance, moisture resistance, insulating properties, and the like.
Especially for use in printed wiring substrates, a trend toward size reduction and increasing functions of electronic appliances has required narrower line pitches and high-density wiring. To meet this trend, the mainstream semiconductor mounting technique shifted from a conventional wire bonding technique to a flip-chip connecting technique with which semiconductor devices and wiring substrates are bonded to each other through solder balls. According to the flip-chip connecting technique, solder balls are placed between a wiring substrate and a semiconductor, and the entire structure is heated to cause solder reflow and achieve bonding; thus, insulating materials for wiring substrates having ever higher heat resistance are in demand. Moreover, the increasing signal speed and frequencies of electronic appliances require further reduction in dielectric constant and dielectric tangent as well as the improvements in heat resistance already mentioned.
Examples of insulating materials in the related art include a benzyl-modified epoxy resin obtained by reacting a phenolic resin and a benzylation reagent in the presence of an acid catalyst, and a benzyl-modified epoxy resin obtained by reacting the phenolic resin and an epihalohydrin (refer to PTL 1). These benzyl-modified phenolic resin and benzyl-modified epoxy resin have low water absorption in boiling water compared to the phenolic resin and the epoxy resin before benzylation; thus, although they have good solder cracking resistance, they exhibit significantly lowered heat resistance evaluated in terms of glass transition temperature. When the dielectric properties of the benzyl-modified phenolic resin and the benzyl-modified epoxy resin were evaluated, both the dielectric constant and the dielectric tangent were high, which did not meet the required levels of dielectric properties of the present date.